Wire electrical cables are usually made in the form that their conductor (core) is drawn through an extruder, which applies an insulative sheath onto the conductor which consists of a suitable plastic material. After the extrusion, the wire cable or the extrudate is guided through a cooling path, a vulcanization path, or a dry crosslinking path. The sheath must have a minimum wall thickness, for reasons of insulation safety for instance. On the other hand, making the wall thickness too thick without necessity is undesirable, for reasons of materials saving or weight. It is therefore desirable to monitor the wall thickness or the diameter of a cable or extrudate, and to provide measures which perform a correction to the thickness when the preset thickness values are exceeded or fallen below.
It is also known to measure the wall thickness of the sheath. When a sheath is mentioned in the following, a single-layer sheath is to be understood for the sake of simplicity. Of course, the insulation of a cable can also consist of multiple layers, which are formed by co-extrusion or by means of several extruders arranged one after the other.
An X-ray measuring device may be suitable for measuring the wall thickness, by way of which it is possible to determine the thickness of the individual layers of a sheath and the diameter of the core. However, it is also possible to determine the thickness of a layer of a sheath by measuring the diameter only, provided that at the same time, the diameter of the core can be assumed to be known or is measured before entering the extruder.
For a given core diameter; the wall thickness of the sheathing depends on the output capacity of an extruder and on the so-called line speed. The output capacity depends primarily on the rotational speed of the feeder screw of the extruder. The line speed is preset by the drive mechanisms which haul the wire cable from a supply drum and draw the wire cable through the extruder and the cooling path to a take-up reel.
As already mentioned, it is known to provide a wall thickness adjustment by measuring the actual value for the wall thickness after the cooling path, with an X-ray device for instance, and comparing the actual value with a preset desired value for the wall thickness. A PI controller outputs a corresponding correcting variable to the extruder, for changing the rotational speed of the screw thereof, for instance. Through this, its output volume is changed also, and consequently the wall thickness of the sheathing. During the start-up phase of the process of applying the insulative sheath, existing extruding machines ramp the line speed up to a preset speed while the feeder screw speed of the extruder follows by a fixed ratio. This empirical method causes various amounts of scrap based on how close the original ratio is to the actual ratio required to produce the specified diameter.
As cooling paths can have a great length, hundred meters and more for instance, such a regulation is slow, of course, in particular when cables with large diameter of the sheathing are produced, which are extruded with line speeds of 10 to 100 meters per minute for instance, at maximum output capacity of the extruder.
The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.